The present invention relates to a liquid crystal display device and more particularly to an active matrix type liquid crystal display device using thin-film transistors.
Active matrix type liquid crystal display devices using active devices represented by thin-film transistors (TFTs) are beginning to find wide use as display terminals for office automation equipment thanks to such features as their thinness, light weight and high display quality, compared to that of a CRT. The display methods of the liquid crystal display device can be classified largely into two categories. In one method, a liquid crystal is held between two substrates each having transparent electrodes and the liquid crystal is activated by a voltage applied to the transparent electrodes to modulate light that has passed through the transparent electrodes and has entered the liquid crystal to display an image. Most of the liquid crystal display products currently in use adopt this method. In another method, the liquid crystal is activated by an electric field virtually parallel to a substrate surface between two electrodes formed on the substrate to modulate the light entering the liquid crystal from a gap between the two electrodes (hereinafter referred to as a lateral field method). This has an advantage of providing a significantly wide viewing angle and is a prospective technique for use in connection with an active matrix type liquid crystal display device. The features of the latter method are described mainly in a Published Japanese Translation of PCT Patent Application from another state No. 505247/1993, Japanese Patent Publication No. 21907/1988 and Japanese Patent Laid-Open No. 160878/1994.
The active matrix type liquid crystal display device described in the above patent application in connection with the lateral field method (also called an in-plane switching method), however, has a response speed of about 100 ms at the fastest, far from the requirement of less than 40-20 ms for the display of dynamic images. Therefore, this type of liquid crystal display device has the drawback that, when a dynamic image is displayed, a residual image is retained making the dynamic image look like a comet with a tail. The response time is defined as the sum of the rise time at the time of voltage application and the fall time at the time of voltage turn-off and will be detailed in the following.
The known lateral field method has the following two types of configuration.
The first known configuration has the liquid crystal molecules in a liquid crystal layer initially orientated in the same direction as the electric field application direction on the interface of the liquid crystal layer on the upper substrate side and, on the interface on the lower substrate side, they are orientated about 90 degrees from the electric field application direction so that the liquid crystal molecules are twisted about 90 degrees when the voltage is turned off. The liquid crystal molecules in this state are turned about 90 degrees in the electric field application direction at the interface on the lower substrate side by an electric field which is almost parallel to the substrate surface generated by two electrodes (referred to as a lateral field) to eliminate their optical rotary power and thereby change their transmittance to display an image.
This configuration, however, needs to rotate the liquid crystal molecules near the interface with the lower substrate by as much as about 90 degrees, and, therefore, the drive voltage may easily become extremely high, higher than 10 v. As for the response speed of this configuration, although the rise time can be made faster to some extent, the fall time becomes at least 40 ms because the liquid crystal molecules must turn back through 90 degrees. This response is not fast enough for displaying a dynamic image.
In the second known configuration the liquid crystal molecules in the liquid crystal layer at the interfaces with the upper and lower substrate are initially orientated almost in the same direction so that they are homogeneously orientated without any twist, when the voltage is turned off. The liquid crystal molecules in this state are wholly rotated about 45 degrees in the electric field direction by the lateral field, when the molecules have a positive dielectric anisotropy. When they have a negative dielectric anisotropy, they are rotated in a direction perpendicular to the electric field to change the birefringent index of the liquid crystal layer at that time and, therefore, to change the transmittance to display an image.
The second known configuration can reduce the drive voltage to about 5 V, i.e. lower than that of the first configuration, which is in a practical range except for rapidly changing, specific dynamic pictures.
In the second known configuration, the response speed depends greatly on the thickness of the liquid crystal layer between the upper and lower substrates and is faster as the liquid crystal layer thickness becomes smaller. When the gap between the substrates is made too narrow, the gap uniformity becomes difficult to maintain, with an increased chance of display variations occurring. There is another problem that the process of injecting the liquid crystal is slow, taking too long a time. Considering these factors, the liquid crystal layer has a practical thickness limit of about 4 .mu.m. The response speed therefore has been some 60 ms at the fastest.
Examples of the first known configuration are described in G. Baur et al, JAPAN DISPLAY 1992, PP.547-550, or by R. A. Soref, Journal of Applied Physics, Vol. 45, No. 12, December 1974, PP. 5466-5468, or by R. A. Soref, Proceedings of the IEEE, December 1974, PP. 1710-1711. An example of the second known configuration is found in ASIA DISPLAY 1995, PP. 577-580 by M. Ohe et al.